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490                              CHEMICAL ENGINEERING
The results of the resistance measurements are usually plotted as the Iogi0# against the temperature, in degrees C. The plots as a rule are approximately straight lines expressed by the relation: LogioR = a  bt, where R = resistance of the specimen in ohms, t = temperature (C.), and a and b are constants. In computing a resistivity factor, based on the dimensions of the cup, the resistance is calculated per cubic centimeter of the material. The criterion used in comparing the merits of each material is the temperature/ Te, at which the material has the definite resistance of one megohm per cubic centimeter, a value used by the electrical division of the Bureau of Standards in evaluating different types of spark plugs.
Types of Refractories.The several types of refractories may be classified from the technical standpoint as follows:
1.  Clay refractories:
(a) High in clay substance.
(6) Low in clay substance, siliceous.
2.  Silica refractories.
(a) Lime bonded. (6) Clay bonded.
3.  Aluminous refractories.
(a) High in alumina.
(6) Aluminum silicates of sillimanite type.
4.  Magnesite refractories.
5.  Dolomite refractories.
6.  Refractories of spinel type.
7.  Carbon refractories.
8.  Silicon carbide refractories.
9.  Special refractories. 10. Insulating refractories.
Clays High in Clay Substance.The purest type of clay is that represented by carefully washed kaolin consisting essentially of a hydrous silicate of aluminum, AhOs. 2Si02.2H20, corresponding to the following percentage composition
Silica .                                . .	HYDROUS, PER CENT .......          46 3	DEHYDRATED, PER CENT 53 8
Alumina                                 . .	39.8	46 2
Combined water ............	..........         13.9	
While this substance, commonly called kaolin, does not correspond to the most refractory mineral combination of silica and alumina found in nature, it is at least the most commonly distributed material, since it may be assumed to be .the fundamental constituent of all fireclays. Other minerals, such as sillimanite, cyanite, and andalusite, corresponding to the general formula Al'/VSiOo, are far more infusible, but are of comparatively rare occurrence in clays.
The so-called melting point of pure clays is close to that of platinum; that is, about 1,750C. Substances whose softening temperatures differ too greatly from that of kaolin should not be considered as fireclays. Though the chemical composition of fireclays appro aches more .or less closely that of kaolinite, Al203.2Si02.2H20, they differ widely as regards their physical structure, varying through all stages from